In a typical cellular radio system, also referred to as a wireless communication system, wireless terminals, also known as mobile stations and/or User Equipment units (UEs) communicate via a Radio Access Network (RAN) to one or more core networks. The wireless terminals may be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “NodeB” or “B node” and which in this document also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
In mobile communications, a wireless terminal can operate in several modes, such as connected mode and idle mode, for example. Moreover, as a wireless terminal travels through a radio access network, the wireless terminal is served by different cells in the course of the travel. That is, as a wireless terminal travels through the radio access network, the wireless terminal is typically handed over, e.g., from one cell to another cell, through a procedure such as a comprehensive cell reselection (when in idle mode) and handover procedures (when in connected mode). This involves large number of “mobility parameters”, which should be properly tuned and optimized to ensure robust mobility performance.
Typically the mobility parameters are configured for ‘normal’ level of user speed. However mobile operators are increasingly deploying networks in wide range of scenarios, where user equipment unit, e.g., wireless terminal, speed may vary considerably. It is therefore important that the desired mobility performance be maintained in different scenarios while user equipment unit (UE) and network complexity is kept within acceptable limit.
Downlink Measurements for Mobility
User equipment unit mobility primarily relies, in both idle and connected modes, on downlink measurements. Downlink measurements used in various systems are described below:
In WCDMA the following three downlink radio related measurements are specified primarily for mobility reasons:
(1) Common Pilot CHannel Received Signal Code Power (CPICH RSCP);
(2) CPICH Ec/No, this is basically the signal-to-noise ratio used for representing the “cell quality” for handover evaluation;
(3) UTRA Carrier Received Signal Strength Indicator (RSSI).
The first two of the above measurements are performed by the user equipment unit on cell level basis on the Common Pilot CHannel (CPICH). The UTRA carrier RSSI is measured over the entire carrier. The above CPICH measurements are the main quantities used for the mobility decisions. In addition, in WCDMA several timing related measurements are also specified for connected mode mobility procedure. They are used to adjust the user equipment timing when performing handover in connected mode.
In E-UTRAN the following three downlink quality measurements are specified primarily for mobility reasons:
(1) Reference Symbol Received Power (RSRP);
(2) Reference Symbol Received Quality (RSRQ);
(3) E-UTRA Carrier RSSI.
The first two of the above measurements are performed by the user equipment on cell level basis on reference symbols. As in case of WCDMA, the E-UTRA carrier RSSI is measured over the entire carrier. The two RS based measurements are indeed also the main quantities, which are likely to be used for the mobility decisions.
Mobility Scenarios
As alluded to above, there are basically two kinds of mobility: (1) Idle mode mobility, which employs or works in conjunction with a cell reselection procedure; and (2) Connected mode mobility, which employs or works in conjunction with a handover procedure. In both idle and connected modes the mobility decisions are mainly based on the same kind of downlink measurements as discussed above.
Both WCDMA and E-UTRAN are frequency reuse-1 systems. This means the geographically closest neighbour cells operate on the same carrier frequency. An operator may also deploy multiple frequency layers within the same coverage area. Therefore, idle mode and connected mode mobility in both WCDMA and E-UTRAN could be broadly classified into three main categories:
(1) Intra-frequency mobility (idle and connected modes);
(2) Inter-frequency mobility (idle and connected modes); and
(3) Inter-RAT mobility (idle and connected modes).
In intra-frequency mobility a UE moves between the cells belonging to the same carrier frequency. This is the most important mobility scenario since it involves less cost in terms of delay due. In addition, an operator would have at least one carrier at its disposal that it would like it to be efficiently utilized.
In inter-frequency mobility the UE moves between cells belonging to different carrier frequencies but of the same access technology. This could be considered as the second most important scenario.
In inter-RAT mobility the UE moves between cells that belong to different access technologies such as between WCDMA and GSM or vice versa.
The cell reselection is mainly a UE autonomous function without the intervention of its serving cell. But to some extent the UE behaviour in this mobility scenario could still be controlled by some broadcasted system parameters and performance specification. The UE in idle mode operates in discontinuous reception (DRX) state with long DRX cycle, e.g., 1.28 second. The DRX ensures power saving but it also adversely affects the measurement performance. Typically UE does measurement at the paging occasions (i.e. at the wake instances at the end of DRX cycle, e.g., once every 1.28 second). Therefore, measurement sampling rate in idle mode is considerably low compared to the connected mode scenario. Due to this reason measurement performance in idle mode becomes much coarser than that would be achievable in connected mode.
The cell reselection procedure in the UE is controlled via several parameters, whose values are signalled by the network or defined in the standard.
As mentioned in the above section that UE in idle mode performs measurements typically at DRX occasions. Thus the measurement period called Tmeasure is considerably longer compared to that in connected mode.
Furthermore in order to save the battery the UE starts measuring the neighbour cells only when the serving cell quality or signal strength falls below certain threshold called Search during certain number of DRX cycles called Nserv. The parameter Search is signalled by the network via broadcast channel in each cell. The parameter Nsery is specified in the standard but may also be signalled by the network.
A UE reselects a new neighbour cell provided it becomes stronger than the serving cell by certain margin called Qhyst over certain time period called Treselection. The parameters Qhyst and Treselection are signal and time hysteresis respectively; both are controlled by the network.
In addition all the above mentioned parameters are specific to one carrier frequency or radio access technology, e.g. WCDMA, GSM. This means if network deploys more than one carrier frequency or radio access technologies then multiple set of cell reselection parameters are signalled.
The above section described only the parameters, which are more relevant to this Solution.
The standard allows UE speeds up to 350 km/hr; this is possible in a high speed train scenario, for example. The UE should be able to reselect cell regardless of its speed up to 350 km/hr. This requires that a UE be able to perform cell reselection evaluation at faster rate at higher speed levels.
The WCDMA system provides support for high mobility state. To realize high mobility state, the parameter “Treselection”, i.e. time hysteresis, can be scaled as a function of speed. The scaling is enforced by the network by signalling a parameter called “speed-dependent scaling factor”. The speed, in turn, is measured in terms of the number of cell reselection, i.e., cell change.
In case of E-UTRAN, a procedure similar to the one used in WCDMA is possible. However, there is on going discussion to further improve the speed-dependent cell reselection procedure as described below.
Cell Reselection Evaluation Processes
In order to realize speed-dependent cell reselection, the UE may perform two simultaneous cell reselection evaluation processes with two different set of values of the cell reselection parameters. In a very simple implementation the UE may perform filtering of the measurements used for cell reselection over two different time windows as shown in FIG. 1. FIG. 1 is an illustration of two parallel, low and high speeds, cell reselection evaluation processes with respective longer and shorter time averaging windows. The same measurement samples are used for both filterings. The shorter time averaging would cater for higher speed at which the fading rate is higher. The longer time averaging would cater for lower speed at which the fading rate is relatively lower. The UE has to employ two independent measurement filtering and cell re-selection evaluation processes running simultaneously. This mechanism can be generalized to multiple speed levels e.g. low, medium and high. In the multiple speeds case there can be three parallel or simultaneous filtering with three different time averaging windows.
In a basic arrangement the multiple parallel cell reselections could be distinguished by different time average windows used for averaging the downlink measurements. However it is also possible that UE uses different values of other cell reselection parameters in the parallel cell reselection evaluation processes. For instance different values of Treselection (time hysteresis), Qhyst (signal hysteresis) and Search could be used for different evaluations in parallel.
Handover Evaluation Processes
Handover, as opposed to cell reselection, is fully controlled by the network through explicit UE specific commands and by performance specification. Handover is performed for the UE in connected mode. In connected mode the UE regularly performs measurements on neighbour cells. The network configures the UE to report events associated with mobility when certain conditions are fulfilled, e.g., when neighbour cell becomes stronger than the serving cell by some margin, i.e. hysteresis, over certain time, i.e., time hysteresis. The UE reports events and/or measurement report to the network, which in turn makes an appropriate decision, e.g. sends a handover command to the UE. Typical connected mode mobility parameters are a layer-3 or higher layer filtering coefficient, for additional time domain filtering, time to trigger i.e. time hysteresis, and hysteresis for signal.
In connected mode a high mobility state is not supported in WCDMA or in E-UTRAN. However, the methodology based on multiple parallel cell reselection evaluation processes can also be used in connected mode. To realize multiple parallel handover evaluation processes to support multiple mobility states in connected mode the typical parameters involved are the layer-3 coefficient, time to trigger (TTT), and hysteresis. It should be noted that layer-3 coefficient, TTT and hysteresis in connected mode are analogous to Tmeasure, Treselection (time hysteresis) and Qhyst (signal hysteresis) in idle mode respectively.
The conventional technology, e.g., existing solutions, as afore described, have problems. The scheme based on multiple parallel cell reselection or handover evaluation processes to realize different UE mobility states (e.g. low, medium, high, etc.) are attractive, but the multiple parallel cell reselection or handover evaluation processes in the UE does involve complexity. The UE has to process, keep track, and store the corresponding measurement results for all multiple processes running in parallel.
In addition, due to shorter time window needed for high mobility state detection, in idle mode or in connected mode when DRX is used the UE power consumption will slightly increase.